A CLOSER LOOK AT REFRACTORS

Achromatic Refractors

Refractors gather more light than reflectors of the same size, but because of their cost mainly,
they are not made to the same sizes as reflectors. So
most refractors won't gather as much
light as big reflectors, but this is not necessary for viewing the planets because they are already
very bright. The
other advantage the refractor has over a reflector is it has no secondary
mirror. A reflector's secondary mirror acts as an obstruction because it's in front
of the telescope's
main mirror. This obstruction degrades the image slightly, especially if the telescope has
a short focal ratio. The shorter the focal
ratio, the larger the secondary mirror is in relationship
to the main mirror.

The first refractors were known for their chromatic aberration. This is where the white light
is broken up into it's different colours it's made up from
much like a rainbow. They are also
known for their spherical aberration. This is where the curvature of the lens causes differences
in distance between the
center of the lens to the focus point, and the outer edges of the lens
to the focus point, once again giving a poor image.

The development of the refractor has brought about lenses of different materials being glued
together which help correct the aberrations. Different
types of materials defract (bend)
light at different angles. This system of gluing the lenses together is called an achromatic lens,
hence the name
achromatic refracting telescope.

Apochromatic Refractors
The lensing system in a apochromatic refractor eliminates just about all aberration. They
are the best telescope you can buy for observing fine detail
in the planets and the Moon. Unfortunately
they are also the most expensive type of telescope per millimeter of aperture (size). Apos,
as they are known,
use multiple elements (up to four) in their main lens made form extra low
dispersion (ED) glass and/or fluorite. Because apos can be corrected for
aberration so
well, they can be manufactured in shorter focal ratios.

Apos, because of their sealed tubes and multi-element glass with fluorite, can take longer
for them to adjust to different temperatures when taking
them from inside to outside. The
different temperature will cause the glass to expand at different rates which will give a poor
image until the glass is the
same temperature as outside.